I had a similar problem once, but there my versions of ‘dev’ were almost, but not quite entirely unlike each other:

One array’s dev was [0:*, …] bu the other array had [0:3212,…] (some large number, and it is unimportant…) therefore it was not open ended any more.
But this is not the case for you… So I am very curious too.

I had to create another array with open ended dimension and merge into it…

I’ve recently had the exact same problem as morgovy. My dimensions are not open-ended and the only difference between the 2 arrays is that the chunk overlaps of the corresponding dimension are different. This happens in 13.6, and I don’t think the dimensions are incompatible because of the chunk sizes.

Another note is that if the 2 arrays are of the exact same bounded dimensions, it is ok as I’ve tried it before.